1. Field of the Invention
The present invention relates to a load detector circuit having a low electric power requirement for use in a load-cell-type electronic weighing scale.
2. Description of the Prior Art
Weighing scales employing electronic circuits for weighing articles have been used in recent years. The weighing devices employing electronic circuits include a load cell comprising a strain gage with its electric resistance variable by a strain caused by the load of an article to be weighed. The strain gage is attached to a strain sensing area of a strain generator. An analog weight signal issued by the load cell is amplified, converted to a digital signal by an A/D converter, and then applied to a display unit for displaying information on the weight, etc.
FIG. 1 of the accompanying drawings shows a load detector of a two-beam-type load-cell electronic weighing scale having four strain gages C1, C2, T1, and T2 attached to a strain generator Y mounted on a support X, for detecting a weight W applied to a scale plate Z.
FIG. 2 shows an electric circuit of the load cell electronic weighing scale. The strain gages C1, C2, T1, and T2 attached to the strain generator Y at positions shown in FIG. 1 are electrically connected to form a full bridge circuit (Wheatstone bridge) having input terminals to which reference voltages Vex.sup.+, Vex.sup.- are applied. When a load is applied to the strain generator and a strain is produced in the strain sensing area, the resistances of the strain gages C1, C2 and the resistances of the strain gages T1, T2 are varied in mutually opposite directions.
The full bridge circuit issued output voltages V.sub.o +, V.sub.o - which are applied to inverting and noninverting input terminals of an operational amplifier. A voltage divider resistor RY is connected to the noninverting input terminal of the operational amplifier, whereas a feedback resistor Rf is connected to the inverting input terminal thereof.
The output voltage (V.sub.o +)-(V.sub.o -) of the full bridge circuit is expressed as follows: It is assumed for simplicity that the resistances of the strain gages are selected to be T=T2=C1=C2=R.
(1) The resistances when the load W is applied to the scale plate Z at a position A are given by: EQU T1=T2=R+.DELTA.R (1) EQU C1=C2=R-.DELTA.R (2)
(2) The resistances when the load W is applied to the scale plate Z at a position B are given by: EQU T1=R+.DELTA.R-.DELTA.Rm (3) EQU T2=R+.DELTA.R+.DELTA.Rm (4) EQU C1=R-.DELTA.R-.DELTA.Rm (5) EQU C2=R-.DELTA.R+.DELTA.Rm (6)
(3) The resistances when the load W is applied to the scale plate Z at a position C are given by: EQU T1=R+.DELTA.R+.DELTA.Rm (7) EQU T2=R+.DELTA.R-.DELTA.Rm (8) EQU C1=R-.DELTA.R+.DELTA.Rm (9) EQU C2=R-.DELTA.R-.DELTA.Rm (10)
In each of the above cases (1), (2), (3), the output voltage (V.sub.o +)-(V.sub.o -) of the full bridge circuit can be determined by: EQU (V.sub.o +)-(V.sub.o -)=(.DELTA.R/R)(Vex.sup.+ -Vex.sup.-) (11)
Where the strain gages are connected as a full bridge as shown in FIG. 2 the combined resistance thereof is expressed by: EQU r={(T1+C2)(T2+C1)}/{(T1+C2)+(T1+C2)} (12)
Therefore, assuming that T1=T2=C1=C2=T, EQU r={(2R).times.(2R)}{(2R)+(2R)}=R (13)
Therefore, the combined resistance of the full bridge is the same as the resistance of each of the strain gages. As a result, the current flowing through the full bridge is large, resulting in increased electric power consumption. Since the bridge consumes a large amount of electric power, it is practically difficult to construct electronic scales of liquid crystal displays and CMOS-ICs and drive then with dry cells.
In order to prevent a weight value from being varied by a change in the resistance of the strain generator Y, a temperature compensating resistor Rs0 may be connected between an output terminal of the bridge and the operational amplifier. With such an arrangement, however, the resistance of the temperature compensating resistor is governed by the resistance of the bridge circuit, and it is tedious and time-consuming to select the resistance of the temperature compensating resistor.